What's the Straight Dope on nickel cadmium batteries?

Factual Questions
1

So I recently bought a cordless(battery powered) hedge trimmer. It came with two nickel cadmium battery packs and a recharger. I know rechargeable batteries can be temperamental so I looked up online what is the best way to care for them and that’s where the confusion begins:

The one thing everyone agrees on is overcharging your nik-cad batteries is bad and will reduce their lifespan.

Some people say undercharging is also bad, others says it’s fine and you should err on the side of caution.

Some say the “memory” effect is overblown and only really reproducible in labs after hundreds of charge-discharge cycles at the exact same voltage, others say it’s a myth, still others say it’s a real problem for the home user.

Some people say you should store your nik-cad batteries with a full charge, some say @ 40% and others say empty.

There seems to be as many pet theories about nik-cad batteries as there are web pages about them! Now I’m sure there had been some scientific studies done so what’s the real deal to extend my batteries useful life span as long as possible?

Thanks!

2

One thing I can guarantee is that you are going to get a whole bunch of conflicting answers.

I once knew a physicist who worked for the MOD researching batteries of all kinds.

He said that they had never been able to demonstrate the memory effect and he believed that it was a myth brought about by people using unsophisticated batter chargers that tried to overcharge batteries.

3

Hmm… I think you should maybe buy some lottery tickets today.

Today’s front-page Straight Dope Classic Column is Do nickel cadmium batteries develop charge memory if not fully discharged?
(Short answer, basically a myth. Also, don’t fully discharge your NiCads if you can help it).

4

Hah, I should’ve checked the front page.

5

Another thing about NiCd cells is their self-discharge tends to be crazy bad. They’re not really the cell of choice if you need power for something you use very infrequently.

6

Battery University

This website seems to be very thorough and rational, I refer to it for my battery questions. YMMV

A lot of cheap nicad chargers seem to have no cut off circuit so leaving a battery in them eventually degrades them. If the battery tends to be hot after it has had lots of time to charge I make sure to manually unplug them.

7

The big problem with Ni-Cad battery packs is that the charger charges them in series (-===+) (= is 1 battery cell). This is bad, because the individual cells are never exactly the same, so they never lost exactly the same charge. Charging them in series, however, forces them to all take the same charge, regardless of how much the individual happens to need. Therefore if you charge so the lowest is fully charged the other two get an overcharge. Charge so the highest is full, and the lowest is undercharged, and runs out first, which can make it take a reverse current, which causes internal shorts that degrade it over time. Charge for the middle one, and the high and low both suffer their respective damage and degradation. No win situation, no matter what you do.

Ni-Cad cells should ideally be charged individually. Ni-MH (the newer type) suffer from these problems to a much lesser degree, because they are less sensitive to overcharge, and they develop internal shorts to a lower degree. Whenever you have a choice, get Ni-MH, even if they cost more, because they will last longer, especially if they are in a battery pack that gets recharged in series.

That’s been my experience (and knowledge I’ve gained from reading electronics experimenter magazines) from several decades of work and home use of both types.

ETA: Be sure the charger says it can be used for the type you have. Ni-MH can be charged in a Ni-Cad charger with little risk, but Ni-Cad can’t be charged in a Ni-MH charger, unless the charger’s label says it can be.

8

Thanks for the replies. Seems clear that really only overcharging and overdischarging are to be avoided.

9

A bit of IMHO, but I loathe the trend of using rechargeable batteries to replace 120 VAC power. Batteries suck, and extension cords are cheap. I always opt for the 120 VAC-powered tools when available.

10

Is it bad to throw Nicads in the trash? I have one I don’t need, and I’d rather not drive out to the local hazmat center just for one battery.

11

yes it is bad.

battery stores, stores that sell rechargeables will often collect for recycling. stores that sell CFL (compact florescent bulbs) also will often collect those for proper disposal.

12

NiCds have been banned in Europe for a while now, as cadmium is toxic and on the list of chemicals restricted under the RoHS legislation; I’m surprised you can still get them in the US.

I agree information on battery charging is confusing and contradictory. Even the battery manufacturers only agree with each other up to a certain point. There’s a reason for this - battery chemistry is complicated, and there are many variables to be accounted for to completely characterise a batterys charging and discharging profiles.

The memory effect is not a myth (sorry Cecil, you’re just plain wrong on this occasion), and I’m with FluffyBob in being fond of the respected BatteryUniversity.com. Here is an explanation of the mechanism behind the memory effect phenomenon. NiMH batteries aren’t as bad as NiCds in this respect though.

Cheshire Human has a valid point - charging cells in series is very much a compromise, and this is when the cells are all from the same batch and have lived identical lives. Mixing cells in this way is definitely a no-no. The problem lies with battery chargers - they are, almost without exception, pieces of shit. Their designs are the product of a race-to-the-bottom, and are priced to compete with disposable alkalines and the like. It wasn’t so bad with NiCd chargers, as they relied on a characteristic hump in the cell voltage to indicate that the cell was reaching full charge, something which was easy, cheap and reliable. This hump is much less pronounced with NiMH cells, and with some types is absent altogether, and so the only proper way to charge them is to monitor the temperature. This is more difficult and expensive to do than might be imagined, and so most chargers either make some sort of attempt to find that elusive hump or have some sort of inadequate temperature monitoring system. Invariably there is a timeout period so the high-current charging is stopped if the voltage or temperature termination criteria aren’t detected.
Small point: You can charge NiCd cells with a NiMH charger, but not vice-versa.

NiCds, and to a lesser extent NiMHs, will benefit from the occasional deep discharge. But beware of discharging a battery pack with the cells in series, as if discharged too low the weaker cells will be overwhelmed by the stronger cells and will be permanently knacked by the phenomenon of “cell reversal”. It is generally accepted that discharging to 1 volt per cell is safe, but no lower.

Some good news on the self-discharge front: A small number of manufacturers have developed NiMH cells with very low self discharge characteristics, so these can now be used in previously verboten applications like smoke alarms, doorbells and remote controls. GP ReCykos are pretty good, but the best by far are Sanyo eneloop (both NiMH). I’ve been working with these in some charger designs, and they really are a major step. The previous generation of eneloops claimed 12 months of useful charge retention; the latest generation now claim 36 months. The charging efficiency of new NiMH designs (not just the Sanyos) is much improved, as evidenced by very small temperature rise during the bulk of the fast-charge period. Older designs got much warmer right from the onset of charging.